1. Field of the Invention
The present invention relates to a fuel cell stack formed by stacking a plurality of fuel cells. Each of the fuel cells includes an electrolyte electrode assembly and a pair of separators for sandwiching the electrolyte electrode assembly. Further, the present invention relates to a method of warming up the fuel cell stack.
2. Description of the Related Art
Generally, a solid polymer electrolyte fuel cell employs a membrane electrode assembly (MEA) which comprises two electrodes (anode and cathode) and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is interposed between separators. The membrane electrode assembly and the separators make up a unit of the fuel cell for generating electricity. A predetermined number of fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. An oxygen-containing gas or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
If the fuel cell has a low temperature at the time of starting operation, power generation can not be performed efficiently. It takes considerable time to raise the temperature of the fuel cell to the desired temperature for power generation. In particular, if operation of the fuel cell is started at a temperature below zero (freezing temperature), water condensation is likely to occur due to the heat radiated outwardly from the fuel cell, and the water produced in the reaction of the fuel cell is not smoothly discharged from the fuel cell. Thus, the desired power generation performance of the fuel cell may not be achieved.
In an attempt to address the problem, the U.S. Pat. No. 5,798,186 discloses a fuel cell system in which a fuel cell stack is connected to an external electrical circuit. Electric current from the fuel cell stack is supplied to the external electrical circuit such that temperature of the membrane electrode assembly exceeds the freezing temperature of water.
FIG. 13 shows the fuel cell stack 1 disclosed in the U.S. Pat. No. 5,798,186. The fuel cell stack 1 includes negative and positive bus plates 2, 3. An external circuit 5 comprising a variable load 4 is electrically connectable to the bus plates 2, 3 by a switch 6.
In the fuel cell system of the U.S. Pat. No. 5,798,186, the temperature of the entire fuel cell stack 1 is raised by self-heating (the exothermic reaction of hydrogen and oxygen within the fuel cell stack 1 and the resistive heating due to internal ohmic losses). If operation of the fuel cell stack 1 is started at a low temperature, a large amount of heat energy is needed for warming up the entire fuel cell stack. It takes a considerably long time for warming up the fuel cell stack 1, and a considerably large electrical energy is required. In particular, if operation of the fuel cell stack is started at a temperature below the freezing temperature, the water produced in the fuel cell stack may freeze undesirably, and power generation can not be performed.